There is much interest in reducing, preferably eliminating VOCs, such as organic solvents, particularly in sprayable coating compositions. One factor which has promoted this interest is the need to reduce atmospheric pollution caused by the evaporation of these solvents during application and cure of coating compositions containing them. Such organic solvents are used in sprayable coating compositions to achieve the desired viscosity for coating applications.
Coating compositions for use in coating transportation vehicles, such as, trucks and automobiles, currently release substantial amounts of VOCs to the atmosphere during application and cure. Since VOCs are typically toxic, smog-producing and noxious, their continued release can cause a detrimental impact on worker safety and the environment. New methods for greatly reducing or eliminating VOCs during the application of such coatings are urgently needed to prevent worker injury and comply with current and pending environmental regulations. Of equal importance is the cost, performance and durability of such coating formulations.
Coating formulations that are used on the surfaces of all types of commercial automobiles and transportation vehicles to protect against the elements and provide an attractive, distinctive appearance include the two-component aliphatic polyurethane systems which provide excellent performance as automotive coatings after drying and cure.
Polyurethanes are of particular interest in high performance coatings because of their combination of useful properties, including flexibility, excellent adhesion to most substrates, abrasion-resistance, UV-resistance, weather-resistance, variable hardness from elastomer to tough plastic, moderate cost, availability as two-component, moisture-cured and aqueous-based, dispersion systems, and low temperature flexibility.
Polyurethanes consist of urethane linkages formed by the reaction of isocyanates with molecules containing hydroxyl groups or other groups with active hydrogens in the presence of catalyst, usually organic tin compounds, such as stannous octoate and dibutyltin dilaurate or tertiary amines, such as 1,4-diazabicyclo(2.2.2)octane (DABCO). The bulk of the polyurethanes used worldwide are based on two aromatic isocyanates: toluene diisocyanate (TDI) and diphenylmethane diisocyanate (MDI). These base materials are occasionally derivatized under controlled conditions to form carbodiimide isocyanates, or reacted with low-molecular-weight glycols to form specialty prepolymers. Such derivatives can simplify materials handling, ease processing and improve foam properties. Other isocyanates such as hexamethylene diisocyanate, isophorone diisocyanate or naphthalene diisocyanate may also be used in some cases for more specialized applications. Compounds with active hydrogens which make up the other half of the polyurethane reaction system, typically consist of poly-oxypropylene glycols (polyether polyols), sometimes supplemented with small amounts of low-molecular-weight glycols such as ethylene glycol or butanediol. In specific microcellular foam applications such as reaction injection molding, a diamine such as diethylene toluenediamine may be used in place of the glycols. In some cases, polyester polyols based on adipic acid and ethylene glycol or butanediol may be used instead of polyether polyols. See, e.g., Modern Plastics, Mid-November, 1993, page 79.
However, polyurethane coating systems currently use large quantities of flammable, toxic and environmentally hazardous organic solvents as diluents to lower the viscosity and make spraying operations possible. In addition, the polyurethane coating systems utilize pure aliphatic diisocyanage as one of the reactive components. The vapors of this volatile material that evolve during coating application and cure are toxic and irritating to workers, and hazardous to the environment. New automotive protective coating systems are required that provide excellent processing and performance without the need for volatile organic components (VOCs) that threaten worker safety and the environment.
A number of coating manufacturers are currently developing low/no VOC coating formulations including, high solids, blocked isocyanate polyurethanes and aqueous-based polyurethane dispersions. However, each of these newly emerging coating formulations possesses certain attributes but not without specific drawbacks. High solids formulations use low VOCs, provide equivalent performance to low solids urethanes, but cannot be applied using spraying techniques. Blocked isocyanate polyurethanes also use low VOCs, have low toxicity levels and good properties. These materials however, must be cured at elevated temperatures (150 to 200.degree. C.) which are not practical when coating automobiles. Aqueous based polyurethanes use no VOCs, are nontoxic and easy to apply via spraying, but to date, have not provided the properties required to meet military specifications, such as adhesion and moisture-resistance. Presently there is no cost-competitive, high performance low/no VOC substitute for current solvent-based polyurethane automotive coating systems.
Polyesters are polymers formed by the reaction of aliphatic or aromatic polyol monomers with aromatic or aliphatic diacids in the presence of catalyst, usually, metallic acetates, such as calcium acetate. These materials have emerged as promising candidates for inclusion as components in advanced aircraft coatings because of their combination of useful properties including: clarity, transparency, absence of color, flexibility, excellent adhesion to most substrates, abrasion-resistance, water-resistance, fuel and oil-resistance, UV-resistance, weather-resistance, variable hardness--from elastomer to tough plastic, moderate cost, low temperature flexibility and availability as variable molecular weight, ester-endcapped, difunctional prepolymers.
Polyester polymers cannot be used as coatings themselves since their functionality as produced is not conducive to the formation of hard, cross-linked coatings. To be used to advantage as advanced coatings, polymers are first functionalized with reactive groups, such as hydroxyl groups, and then cured by chemical reaction with other comonomers, such as polyurethane diisocyanate monomers, into hard, tough polymer resins that can be used as protective coatings. Hydroxy-functional, polyester-based reactive prepolymers are frequently selected for use as components in polyurethane coatings over polyether and other type, polymers since they impart superior solvent-resistance, mechanical toughness and clarity to the coating and do not yellow upon exposure to intense UV light.
The major problems associated with the use of polyester-derived polyurethanes have involved the requirement for addition of substantial quantities of flammable, toxic solvents to polyurethane coating formulations to lower viscosity sufficiently for spray-painting operations. Another problem involved the use of toxic, irritating and volatile diisocyanate monomer components as major components in the polyurethane coating. The vapors that evolved from polyurethane coating operations could cause serious injury to workers breathing the fumes if they were not wearing protective equipment.
A number of coating manufacturers have been actively pursuing the development of high-performance low VOC and no VOC coating formulations. To date however, none of these coatings has exhibited the required processibility or properties for use in e.g., automotive coatings.
Accordingly, sprayable polyester/polyurethane based coating compositions which contain no volatile organic components yet provide coatings having useful properties are desired.